1. Field of the Invention
The present invention relates to a trenched MOSFET structure with a guard ring and a channel stop and a method for manufacturing thereof, and more particularly to a structure of a trenched MOSFET which solves current leakage, and a method for manufacturing the same.
2. The Prior Arts
In the structure of a trench Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) or vertical transistor, the gate of the transistor is formed in a trench on top of a substrate and the source/drain regions are formed on both sides of the gate. This type of vertical transistor allows high current to pass through and channel to be turned on/off at a low voltage.
Referring to FIG. 1, a cross-sectional diagram of the structure of a trenched MOSFET is shown. An N-type doping epitaxial region 105 is provided on a N+ substrate 100. A plurality of trenches 106 and a trench 107 are formed on the N-type doping epitaxial region 105 that having lower doping concentration than the substrate 100, and the trench 107 is wider and deeper than the trenches 106. The surface of trenches 106 and the trench 107 which are covered a gate oxide layer 110 thereon are filled with a polysilicon layer to form a plurality of trenched gates 115 and a wide trenched contact gate 116 respectively. A plurality of P-type doping regions 120 are formed on both sides of the trenched gates 115, and a P-type doping regions 120a is formed on the other side from the P-type doping regions 120 of the wide trenched contact gate 116. A plurality of N+ doping regions 125 are formed in the P-type doping regions 120, and the N+ doping regions 125 are used as the source regions of the MOSFET structure. A metal layer 160 is formed on the top of the MOSFET structure and is formed as the source metal, the gate runner, and the field plate metal of the MOSFET. An insulating layer 130 is formed under the metal layer 160 for insulating from the trenched gates 115 and the wide trenched gate 116, and the contact plugs 137 are formed in the P-type doping regions 120 and the wide trenched gate 116 for gate contact. The contact plugs 137 been the metal connections of the MOSFET structure respectively contact the doped polysilicon at the bottoms of the trenches 106 and the trench 107 without shorting to the P-type doping regions 120 and are penetrated through the insulating layer 130 to contact with the metal layer 160. A plurality of P+ heavily-doped regions 121 are formed at the bottoms of the trenched gates 115. The MOSFET structure of the prior arts also has a P-type guard ring 170 which is formed aside the P-type doping regions 120a underneath the field plate metal of the metal layer 160 of the MOSFET to increase breakdown voltage in termination. However, the structure in FIG. 1 has low breakdown voltage occurring on trench bottom of the wide trenched gate 116 as result of wider trench which has deeper trench depth than the trench depth in active area. The trench depth is deeper when the trench width is wider because more open area allows more etching gas goes into trench during dry etching silicon process. When reverse bias between drain and gate/source increases, avalanche will first occur on the trench bottom of the contacted trenched gate 116 because it has deeper trenched gate.
As said above, the avalanche early occurs near trench contacted gate due to deeper trench than trench gate in active area as result of bigger CD of trench contacted gate than the trench gate in active area. The trench contacted gate is wider than trench gate in active to allow enough space for trench gate contact without shortage source area. BV instability in termination due to high epi resistivity easily causing net positive charge at interface between dielectric and silicon layer induced by negative charge in dielectric layer. A leakage path 190 is formed as shown in FIG. 1 below.
The present invention provides a new structure of trenched MOSFET structure with a guard ring wrapped around the contacted trenched gate which improves the lack of the prior art.